Instead of silently defaulting to SITEID when courseid (coming
from message_send()/\core\message\manager::send_message()) is missing,
now a debugging message is shown to allow developers to fix their
messages to, always, include courseid.

Raw creation of events via message_sent::create() missing other[courseid]
leads to coding exception since now (there shouldn't be any legacy use, as far as
they are always created via create_from_ids() when sending a message.

Updated upgrade.txt notes a little bit, added references the 3.6 final
deprecation issue (MDL-55449) and covered with unit tests.